The present invention relates to apparatus for mixing a dry particulate material and a liquid diluent, and relates particularly to the mixing of dry particulate materials which are difficult to wet or disperse, are shear sensitive or tend to produce a viscous suspension. The invention has particular application to the preparation of mixtures of dry synthetic polyelectrolytes and water.
Polymers are commonly used in water treatment equipment in order to remove solids suspended in the water. Polymers or polyelectrolytes as they are sometimes called carry an electrostatic charge which attracts particles suspended in water. Since virtually all solids carry a negative or positive charge, they are attracted to these polymers. Polymers have extremely large molecules with millions of charge sites that attract suspended particles. Synthetic polymers are available in dry and liquid form. Dry polymer is desirable for many applications because it has low weight, which saves on shipping expenses; can be easily stored and shipped in plastic lined sacks, which are relatively inexpensive as compared with disposable metal drums which must be used for liquid polymer, and has indefinite shelf life, whereas with liquid polymers the more dilute the mixture the shorter the shelf life. Furthermore, dry polymers have been approved as safe and effective in certain food grade and potable applications, whereas many liquid products have not received such approval.
However, dry polymer must be mixed with water before it can be used. The dry polymer is hygroscopic and its suspensions in water are thixotropic. In other words, the dry polymers do not readily mix with water. Most existing systems for mixing dry polymer and water rely on two steps, viz., (1) a wetting/dispersing step wherein the dry polymer is initially contacted by and mixed with the water, and (2) a mixing/aging step wherein the mixture is further mixed and stored in a holding tank.
The first step is generally accomplished by some type of eductor or vortex device to impart a high energy into the dry material and to get the individual particles thereof separated and dispersed as quickly as possible to prevent them from agglomerating into clumps, fisheyes, stringers, snowflakes, and the like which, once formed, are difficult to eliminate.
Nevertheless, many of the existing mixing systems are subject to agglomeration of dry polymer particles during the wetting/dispersing step. Furthermore, the dry polymer is typically introduced into the eductor through a straight tube which frequently is bridged by dry polymer, further aggravating the agglomeration condition. Additionally, unwetted particles which leave the eductor frequently become stuck along the inlet pipe to the holding tank, causing clogging at that location. Many of these systems also rely on the introduction of air in in the wetting/dispersing step to facilitate separation of the particles of dry polymer. But this reduces the volume of water which is available in the wetting/dispersing stage of the process.
The second step is generally accomplished in a holding tank equipped with a propeller type mixer. The wetted and dispersed polymer/water mixture is introduced into the tank through a pipe at a fixed location, the stationary propeller being used to accomplish distribution and agitation of the incoming material as the level in the tank rises. In many of these prior systems additional water is fed directly to the holding tank for further dilution of the mixture.
But this arrangement does not achieve an even distribution of the mixture in the holding tank, and the mixing energy imparted by the impeller can vary depending upon the level of mixture within the holding tank. Furthermore, the introduction of additional diluent into the holding tank means that that additional diluent is not available for use in the initial wetting and mixing step.
Furthermore, in prior mixing devices severe agglomeration problems can result in the event of a stoppage of the water supply in the initial mixing and wetting step, since the feeding of dry polymer may continue for a short time until the feed mechanism can shut down.